1) Field of the Invention
The present invention relates to a spark plug in which at least one of mutually confronting portions of a center electrode and an earth (ground) electrode which are in opposed relation to each other is constructed with a noble metal chip, and more particularly to a spark plug for use in a gas engine and the like, suitable for use at high temperature.
2) Description of the Related Art
In general, a spark plug is made up of a center electrode, an insulator located outside the center electrode, a housing located outside the insulator and an earth electrode whose one end portion is joined to the housing and whose other end portion is disposed to be in opposed relation to the center electrode in a state where a spark discharge gap is interposed therebetween, and it is employed as an ignition device.
In addition, for the purpose of improved durability and the like, there has been proposed a technique in which at least one of the confronting portions of the earth electrode and the center electrode is made in the form of a noble metal chip joined to an electrode base (matrix) material (for example, see Japanese Patent Laid-Open Nos. HEI 9-92432 and 2002-83663). In general, as a noble metal chip establishing a confronting portion between electrodes, there has been employed a Pt (platinum) alloy chip or an Ir (iridium) alloy chip.
In general, the Pt alloy chip is joined to an electrode base material by means of resistance welding, while the Ir alloy chip is joined to the electrode material through the use of the laser welding.
Meanwhile, in general, in the case of the Ir alloy chip joined to the electrode base material by the laser welding, its joint (bonding) reliability at high temperatures is lower than that of the Pt alloy chip joined to the electrode base material by resistance welding.
For example, the Ir alloy chip joined by the laser welding secures the joint reliability when the electrode temperature does not reach approximately 900° C., difficulty is encountered in reliably securing the joint when the electrode temperature exceeds this temperature. On the other hand, the Pt alloy chip joined by the resistance welding can provide sufficient joint reliability even if the electrode temperature exceeds approximately 900° C.
For this reason, in the case of a spark plug to be used in a high-temperature atmosphere where the Ir alloy chip cannot secure the joint reliability, the Pt alloy chip has been employed in order to secure the reliability of the joint between the noble metal chip and the electrode base material.
For example, in the case of a spark plug for use in a gasoline engine, the earth electrode temperature is below 700° C. in the normal use, whereas for a recently developed gas engine, such as cogeneration, the earth electrode temperature becomes as high as approximately 850° C. to 950° C. in normal use. For this reason, for the earth electrode of a spark plug for use in a gas engine, a Pt alloy chip has been employed as a noble metal chip having a high reliability at high temperatures.
However, although the Pt alloy chip is superior in the reliability of the joint with the electrode base material, platinum grains develop in high-temperature atmosphere, which creates a problem in that the platinum grains extend as a globular structure in the spark discharge gap and cause a short-circuit between the electrodes. Referring to FIGS. 5A and 5B, a description will be given hereinbelow of a concrete example examined by the present inventor about this problem.
FIGS. 5A and 5B illustratively show a discharge portion and a portion around it and are illustrations of the results of the experiment and observation by the present inventor, and FIG. 5A is an illustration of an initial state while FIG. 5B is an illustration of a state of the growth of platinum balls along with the use in a high-temperature atmosphere in a gas engine.
In this case, in a using state of the gas engine, since the temperature of a center electrode 30 is below 900° C., a confronting portion of the center electrode 30 is made as an Ir alloy chip 32 laser-welded to an electrode base material.
On the other hand, since the temperature of an earth electrode 40 is approximately 850 to 950° C., a confronting portion of the earth electrode 40 is made with a Pt alloy chip J43 resistance-welded to the electrode base material. A spark discharge gap 50 is defined between the Pt alloy chip J43 and the Ir alloy chip 32. In the case of a spark plug for a gas engine, this spark discharge gap has a dimension of approximately 0.15 mm to 0.6 mm.
Concretely, the earth electrode 40 side Pt alloy chip J43 is made of Ir of 20 weight %, with the remains being Pt. That is, it is made of Pt-20 Ir. The base material of the earth electrode 40 and the center electrode 30 is made of a Ni (nickel)-base alloy, such as Inconel (registered trademark).
As FIG. 5A shows, in the initial state, the interior of the earth electrode 40 side Pt alloy chip J43 almost entirely has a layer-like configuration. On the other hand, as FIG. 5(B) shows, along with the use, a discharge surface side portion of the Pt alloy chip J43 recrystallizes so that platinum crystal grains 70 separate, thereby changing from the layer-like configuration to a globular structure.
At the occurrence of this transformation, due to the influence of the discharge sputter, the platinum grains 70 grow up, and separate from the discharge surface of the chip J43 to grow up along a discharge path indicated by an arrow in FIG. 5B. When the grown-up platinum grains 70 reach the confronting center electrode 30 side Ir alloy chip 32, the short-circuit occurs between both the electrodes 30 and 40.
In particular, in the case of a spark plug for a gas engine, since the required discharge voltage is high, there is a need to set an initial spark discharge gap at as a smaller value as approximately 0.15 to 0.6 mm, as compared with an initial spark discharge gap (for example, approximately 1 mm) of a spark plug of a gasoline engine. Therefore, the aforesaid grown-up platinum grains 70 facilitate the occurrence of the short-circuit between the electrodes 30 and 40.